Starter spins but no crank

[Burg guy]

On an automatic trans it's a flex plate. The engine stops at the same spot on the flex plate/ flywheel every time thus once the teeth are shot on that spot your are done. Your Bendix is stuck and not engaging the ring gear. 3 bolts hold the starter on plus the battery cable. Not the easiest job but can be done. Take it to a shop that rebuilds starters and it will be ready to reinstall.

 

[Espos4]

As far as automatic transmissions go, You can also have a flex plate with no teeth on it. The flex plate bolts up to the end of crankshaft. The torque converter actually has the teeth on it. That gets bolted to the flex plate.
This is a design used by Ma Mopar.